A capacitor has two parallel plates separated by a non-conductive medium such that electric energy can be stored by receiving a positive charge on one plate and a negative charge on the other. The storage capability of the capacitor is related to the capacitance C by the formula:U=½CV2where U is the potential energy, C is the capacitance and V is the potential difference between the plates. The capacitance is then related to the area of the plates by the formula:C=ΣA/dwhere Σ is the dielectric constant of the media separating the plates, d is the distance between the plates, and A is the surface area of the plates. By combining these two formulas it can be seen that the energy storage capabilities of a capacitor are increased by reducing the distance between the plates and increasing the surface area of the plates. The minimum distance between the plates is limited by the insulating qualities of the dielectric material. The capacitor will fail if the charge on the plates exceeds the insulating capacity of the dielectric material and arcing occurs between the plates. The only limit to increasing the area of the plates is the practical limitation of size. A capacitor capable of retaining massive amounts of electric potential energy might require an unacceptably large volume of space.
The charge of a capacitor is stored on the surface of the electrodes, and a super capacitor, that is, a capacitor which can retain many times the potential energy of a convention capacitor, can be made by providing electrodes having surfaces configured to provide enhanced surface area for receiving a charge. Such electrodes have porous surfaces, which by virtue of the irregularities of the pores, have greater surface area and therefore can retain a greater number of electrons thereon.
There are many methods of forming a porous surface on an electrode. One common method is to suspend metal oxide particles in a slurry into which the electrode is dipped, leaving a coating of metal oxide on the surface thereof. After the coating is applied, the electrode is heat treated to bond the coating to the surface of the electrode. The electrode can then be dipped in the slurry again to apply a second layer of the metal oxide, thereby thickening the coating of metal oxide on the surface of the electrode. By applying multiple layers, the thickness of the coating is increased thereby increasing the effective surface area of the electrode and correspondingly increasing the capacitance of the capacitor into which the electrode is incorporated. A super capacitor having electrodes with porous surfaces as described above will have an exponentially larger capacitance than a capacitor of corresponding size.
While super capacitors are highly desirable, the multiplicity of steps needed to develop a sufficiently thick layer of porous electrically conductive material, especially the heat treatments following each dipping process, are costly. It would be desirable, therefore, to provide a less expensive method of manufacturing an electrode for a super capacitor.